1. Field of the Invention
The present invention relates to an organic composite-plated steel sheet provided with a thin film coating thereon, more particularly to a method for producing the organic composite-plated steel sheet prevented from liberation of chromium and high in corrosion resistance.
2. Description of Related Art
Recently, reduction of cost and enhancement of the rust resistance of zinc- or zinc alloy-plated steel sheets (hereinafter referred to as "zinc alloy-plated steel sheets") has been highly desired by the fields of automobiles and appliances, and researches for the development of new products have been increasingly conducted.
In the production of the organic composite-plated steel sheets by applying a thin film coating on the above-mentioned zinc plated steel sheet which has been chromated, the chromating has been required to be carried out by coating.
In order to meet this requirement, it is necessary to develop a hardly soluble chromate coating type treatment technique which inhibits chromium from liberation from the coated chromate film.
A proposal to make the chromate film hardly soluble has been reported in JP-A-50-158535.
This patent application discloses a chromating solution of a chromic anhydride--phosphoric acid--water-soluble or water-dispersible polymer compound type, in which at least 70% of hexavalent chromium ions have been reduced with a reducing agent such as ethylene glycol. However, the chromate film produced by this technique contains polymers and so, is inferior in weldability though superior in hard-solubility, corrosion resistance and coatability.
The chromate solution disclosed in JP-B-61-58552 is of a chromic acid--chromic acid reduction product--silica sol type. However, there is the problem that when the surface-treated steel sheet having the chromate film formed thereon is worked and coated, mainly hexavalent chromium ions in the chromate film are readily liberated by washing with water before the coating, resulting in reduction of the corrosion resistance of the film.
Furthermore, use of silane coupling agents for reduction of hexavalent chromium ions in the chromating solution is disclosed in JP-A-58-22383 and 62-83478. Films formed by the processes mentioned therein are all excellent in adhesion, but the chromate films formed by the process mentioned in the former patent application are inferior in the alkali resistance, because they are formed with chromating solution containing no phosphoric acid and the chromate films formed by the process mentioned in the latter patent application are also inferior in the alkali resistance.
Properties of respective components in the above-mentioned conventional chromating processes were examined to find that organic polymers and silica have tendencies to enhance the corrosion resistance but deteriorate the alkali resistance, and silane coupling agents have a tendency to reduce hexavalent chromium ions in the chromating solution to cause transient decrease in the hexavalent chromium ion concentration of the chromating solution and to make the corrosion resistance of the chromate film unstable.
As mentioned above, various processes have been disclosed, but in all of them, the properties of the chromate films depend greatly upon drying conditions in the chromating process
Furthermore, it has been proposed in JP-B-55-51032 and JP-A-59-162278 and 61-584 to increase the rust resistance by coating an aqueous solution of a mixture of a water-dispersible emulsion resin with chromic acid or a chromate compound of a high rust resistance on the surface of zinc-plated or zinc alloy-plated steel sheets. However, all of the resulting films are high in rust resistance, but sweating or liberation of chromium with an aqueous treating solution are seen. Thus, there are some practical problems. Moreover, JP-A-60-50181 and 60-149786 propose to carry out the thin film coating of a blend of a water-dispersible emulsion resin and an organic composite silicate (silica sol, a silane coupling agent or the like). These processes do not offer the problems of the liberation of chromium, but they have problems in the properties of the coated film, especially water resistance, because the film contains silica in the form of stable colloid and so alkali ions or ammonium ions. Furthermore, solvent type coating compositions containing fine silica is very high in the viscosity owing to their structural viscosity, and coating of a uniform thin film is impossible. It can be considered to rupture the structural viscosity by the hydrogen bond of alcohols to reduce the viscosity, but this cannot be applied to systems using a polyisocyanate compound as a curing agent.
As mentioned above, the thin film coated rustprevented steel sheets require 1 higher corrosion resistance, 2 further improvement in adhesion to a coating (especially, cationically electrodeposited film) formed on the steel sheets, and 3 reduction of baking temperatures (140.degree.-170.degree. C.) for improvement of the working strength of the steel sheets. Various coating compositions have been proposed in order to meet such requirements. The above requirements are especially important to the coating systems using polyisocyanate compounds as curing agents. Since the proposed coating compositions have the problem of thixotropy, there have not yet been obtained any practically usable, organic solvent-soluble epoxy resin coating compositions as the solvent-type compositions or coating compositions containing polyisocyanate compounds or silica powders for the thin film type rust-prevented steel sheets.